Add .before and .after arguments to mutate

[eutwt]

Example with this PR loaded:

library(dtplyr)
library(dplyr, warn.conflicts = FALSE)

lazy_dt(data.frame(x = 1, y = 1)) %>% 
  mutate(z = 1, .before = y)
#> Source: local data table [1 x 3]
#> Call:   setcolorder(copy(`_DT1`)[, `:=`(z = 1)], c("x", "z", "y"))
#> 
#>       x     z     y
#>   <dbl> <dbl> <dbl>
#> 1     1     1     1
#> 
#> # Use as.data.table()/as.data.frame()/as_tibble() to access results

^{Created on 2021-08-29 by the reprex package (v2.0.1)}

[hadley]

In general, I don't think dtplyr needs to provide experimental arguments, but I'm pretty sure that all the current dplyr experimental arguments are going to stay, so this still makes sense. Otherwise looks good, just one small styling suggestion and it needs a news bullet 😄

[hadley]

Indenting is slightly off here and below.

[eutwt]

Thanks, fixed.

[eutwt]

Should be all set now

[hadley]

BTW we add news bullets to the top, since it makes resolving the merge conflicts a little easier.